This invention relates to a torque transmission device which is used e.g. between the drive shaft and the wheels of a vehicle to transmit and cut of the driving torque.
When a vehicle is turning a corner, the turning radius of the front wheels is greater than that of the rear wheels. Thus, if it turns a tight corner with the front and rear wheels coupled directly, since the front wheels tend to rotate faster, they will slip, developing a phenomenon as though they were being braked.
In a conventional four-wheel drive vehicle, due to such a braking phenomenon, when turning a tight corner or while travelling on an urban street, a driver had to select either two-wheel drive mode or four-wheel one by releasing or regaining the coupling between the Front and rear wheels. Such a mode changeover was troublesome for a driver.
in contrast, as shown in FIG. 12, a full-time four-wheel drive vehicle is known in which a torque transmission device A in the form of a viscous coupling is disposed between a drive shaft C branching from a transfer B of the engine and the front differential E provided on the front axle D to absorb any difference in rotation between the Front and rear wheels by utilizing the resistance of a high-viscosity fluid contained in the viscous coupling.
However with a viscous coupling in which a turning torque is transmitted with the resistance of a high-viscosity fluid, the efficiency of torque transmission is low due to losses when resistance is produced. Also, since the shear resistance is small while the difference in rotation is small, it cannot transmit a sufficiently large torque for the weight of the vehicle.
In order to transmit a larger torque, it is necessary to increase the number and area of discs for shearing the high-viscosity fluid. This makes tile drive system bulky. Further, since the shear resistance of the high-viscosity fluid is large while running at low speed, a dragging torque may be produced while turning at a low speed. Thus, the braking phenomenon while the vehicle is turning on a tight corner cannot be wiped out.